Recently, there have been increased opportunities of using electrophotographic copiers or printers in the field of printing or color printing. There is a strong trend of requiring high quality digital black-and-white or color images in such fields of printing or color printing. In response to such a requirement, there has been proposed formation of high precision digital images by use of a short wavelength laser light (Patent Documents 1 and 2). However, the current condition is that even when forming a precise electrostatic latent image on an electrophotographic photoreceptor by use of a short wavelength laser light and reducing the exposure diameter, the finally obtained electrophotographic image does not achieve sufficiently high image quality.
The cause thereof is due to the fact that there were not sufficiently addressed newly generated problems in images obtained by imagewise exposure at relatively short wavelengths.
As a first problem, in a conventional photoreceptor developed for a longer wavelength laser light, light transmittance for a shorter wavelength laser light has not been fully enough, whereby no excellent sensitivity characteristics has been obtained. This is due to the light absorption range of a charge transport material extended to a portion of 400 nm or more. As a result, no uniform picture image cannot be obtained, for example, due to the effect of an uneven film thickness. On the other hand, a charge transport material which does not substantially have absorption in 400 nm or more had a problem in image stability because of the inferior light stability of the charge transport material. It was found by the present inventor that this problem can be overcome by employing a prescribed charge transport material as disclosed in Patent Document 3.
As a second problem, the transfer current increases as the diameter of the toner is decreased in order to obtain a higher quality image employing a short wavelength laser. When the transfer current is increased, a problem of transfer memory tends to occur in the photoreceptor. Specifically, in a photoreceptor employing a phthalocyanine pigment as a charge generation material, the transfer memory is easy to occur.
Patent Document 1Japanese Patent Application Publication Opento Public Inspection (hereafter referred to as JP-A)No. 2000-250239Patent Document 2JP-A No. 2000-105479Patent Document 3JP-A No. 2007-108314